A wide variety of implantable medical devices for delivering a therapy or monitoring a physiologic condition have been clinically implanted or proposed for clinical implantation in patients. In some cases, implantable medical devices deliver electrical stimulation therapy and/or monitor physiological signals via one or more electrodes or sensor elements, which may be included as part of one or more elongated implantable medical leads. Implantable medical leads may be configured to allow electrodes or sensors to be positioned at desired locations for sensing or delivery of stimulation. For example, electrodes or sensors may be carried at a distal portion of the lead. A proximal portion of the lead that may be coupled to an implantable medical device housing, which may contain electronic circuitry such as stimulation generation and/or sensing circuitry.
For example, implantable medical devices, such as cardiac pacemakers or implantable cardioverter defibrillators, provide therapeutic stimulation to the heart by delivering electrical therapy signals, such as pulses or shocks for pacing, cardioversion or defibrillation, via electrodes of one or more implantable leads. In some cases, such an implantable medical device may sense for intrinsic depolarizations of the heart, and control the delivery of such signals to the heart based on the sensing. When an abnormal rhythm is detected, which may be bradycardia, tachycardia or fibrillation, an appropriate electrical signal or signals may be delivered to restore the normal rhythm. For example, in some cases, an implantable medical device may deliver pacing, cardioversion or defibrillation signals to the heart of the patient upon detecting ventricular tachycardia, and deliver defibrillation electrical signals to a patient's heart upon detecting ventricular fibrillation. Pacing signals typically have a lower energy than the cardioversion or defibrillation signals.
Patients with heart failure are, in some cases, treated with cardiac resynchronization therapy (CRT). CRT is a form of cardiac pacing. In some examples, CRT involves delivery of pacing pulses to both ventricles to synchronize their contraction. In other examples, CRT involves delivery of pacing pulses to one ventricle to synchronize its contraction with that of the other ventricular, such as pacing the left ventricle to synchronize its contraction with that of the right ventricle. CRT is one example of a variety of modes cardiac pacing in which stimulation is delivered to one chamber or location at a time that is an interval before or after an event at another chamber or location. The event at the other chamber or location may be the delivery of a pacing pulse to the other chamber or location, or the detection of an intrinsic cardiac depolarization at the other chamber or location.